1. Field of the Invention
The present invention relates to a device for manufacturing molded articles of plastics material which have an outer skin preferably of a plastics material free of expanding agents and a core, preferably a porous core of plastics material containing an expanding agent. The device includes an extruder head with a main duct in communication with a discharge opening. Two feed ducts for the two plastics materials lead angularly offset into the main duct at locations axially spaced from the discharge opening and from each other. A closing needle is placed axially movable in the main duct. The two feed ducts can be separated from the main duct by means of the closing needle. The closing needle defines a connecting duct extending essentially in axial direction. The connecting duct ends, on the one hand, at the end face of the closing needle and, on the other hand, at the circumference of the closing needle radially toward the more remote feed duct.
2. Description of the Prior Art
An extruder head of the above-described type is already known from German Offenlegungsschrift No. 3,247,000. This extruder head is characterized in that, in the closed position of the closing needle and seen in axial direction from the discharge opening, the connecting duct provided in the closing needle emerges radially from the closing needle in front of the more remote feed duct. This extruder head is additionally characterized in that the distance between the front edge of the inner radial outlet opening of the connecting duct and the free end face of the closing needle is greater than the smallest axial distance between the front edge of the front feed duct and the front edge of the rear duct, on the one hand, and smaller than the greatest axial distance between the front edge of the front feed duct and the rear edge of the rear feed duct, on the other hand.
Compared to other known extruder heads as disclosed, for example, in German Utility Model No. 8,034,352, the extruder head according to German Offenlegungsschrift No. 3,247,000 has the advantage that in an extruder head which is structurally as complicated, it is possible during the injection procedure to effect a continuous transition from one plastics material to the other plastics material and vice-versa, without requiring the use of special locking devices. Moreover, it is possible, if necessary, to inject into a mold a plastics material without expanding agent as well as a plastics material containing an expanding agent separately from each other without the use of special locking devices.
The device of the type described above for manufacturing molded articles from plastics material is deficient in that, when an injection molding procedure is carried out with the plastics material containing an expanding agent, a so-called afterpressure can be exerted on the plastics material already in the mold only if an intermediate stop means is provided in the feed range for the plastics material with an expanding agent, i.e., if the extruder head is operated with a special control in order to achieve sealing.
It has been found that, due to the sluggish movements resulting from this type of control, the reproducibility of the desired result, i.e., sealing, is not always ensured.
Another deficiency of known devices for manufacturing molded particles of plastics material as disclosed, for example, in German patent No. 2,342,789, is that, due to the subsequent injection with plastics material free of expanding agents, the respective closing needle must press the plastics material in front of it into the sprue. While this can be done easily in foamed molded articles having relatively thick walls, this cannot be achieved in molded articles having thin walls because the sprue has usually at this time already hardened.
It is, therefore, the principal object of the present invention to further develop a device for the manufacture of molded articles of plastics material of the above-specified type in such a way that, in a device of inexpensive construction, the subsequent injection with plastics material free of expanding agents is made possible and the main duct can be closed by means of the closing needle in such a way that only small volumes of melt must be moved. In addition, the extruder head is to have optimum rheological properties. Another object is to make it possible to operate plastics material flows having different temperatures and different types of plastics material, for example, plastics material free of expanding agents and plastics material containing an expanding agent.